Dataform readers and methods

ABSTRACT

Dataform readers and methods provide operation with a sequence of overlapping exposure periods for successive lines of sensor elements of an array. Also, the exposure periods for successive lines of sensor elements, or successive subsets of lines, can be independently determined. In operation of a dataform reader illumination of a target area is turned on before exposure of the first line of sensor elements, each line of sensor elements is exposed by reflected illumination in an exposure period which overlaps with the exposure period of one or more other lines, and illumination is then turned off. Using a CMOS construction, for example, the level of image signals read from one line of sensor elements is referred to a look-up table to determine an appropriate exposure period for a subsequent line or lines of elements of an array. This process is repeated to achieve exposure periods adjusted for localized exposure conditions for each successive line or subset of lines of sensor elements.

This is a continuation of application Ser. No. 08/332,592, filed Oct.31, 1994 now U.S. Pat. No. 5,521,366, which is a continuation-in-part ofapplication Ser. No. 08/280,489, filed Jul. 26, 1994 now U.S. Pat. No.5,572,006.

This invention relates to systems and methods for reading dataforms,such as bar codes and matrix codes, and to such systems operating withoverlapping exposure periods for successive lines of sensor elements ofan array and having automatic exposure control implemented forsuccessive lines or subsets of lines of sensor elements.

BACKGROUND OF THE INVENTION

While a variety of types of sensor array scanners have been provided forreading more complex forms of two dimensional bar-codes and matrixcodes, these sensor array scanners are all continuous frame scanners andtherefore suffer from high power consumption. Because many sensor arrayscanners are portable and powered by batteries, there exists a need fora portable reader with single frame capability and therefore reducedpower consumption and correspondingly extended battery life. There alsoexists a need for a portable reader with enhanced accuracy andreliability, as well as reduced size and light weight.

In such a portable reader it is further desirable to provide forincreased imaging accuracy by enabling exposure control to beaccomplished on the basis of individual lines of sensor elements orsuccessive limited subsets of lines of sensor elements of an imagingarray.

BACKGROUND OF DATAFORMS

The application and use of bar codes and matrix codes are well known andgrowing. Bar codes and matrix codes are forms of "dataforms", which forpresent purposes are defined to include all arrangements whereby data isfixed in some form of machine readable copy. Thus, dataforms include oneand two dimensional bar codes, matrix codes and graphic codes, as wellas words and numbers and other symbols, which may be printed or etchedon paper, plastic cards and metallic and other items. Dataforms may beprinted in invisible ink, magnetically recorded via magnetic stripes ormagnetic ink fonts, electromagnetically recorded via RF tags, engraved,stamped, tattooed (on skin), formed by ion doping (for semiconductorwafers) or biochemical binding, etc.

In the utilization of dataforms, data originally encoded is recoveredfor further use in a variety of ways. For example, a printed bar codemay be optically scanned to derive reflectance values which aredigitized, stored in buffer memory and subsequently decoded to recoverthe data encoded in the bar code. Regardless of the particular type ofdataform, an image is typically acquired and stored as pixel values forfurther processing. An image of a bar code or matrix code existing as agraphic image can be acquired by use of a CCD scanner, a laser scanneror other suitable device which is capable of distinguishing betweendifferent reflective values of light reflected from a dataform. Thus,for example, a bar code typically comprises black or dark colored bartype elements printed on a white or light colored background area, withwhite or light colored spaces between the elements of the bar code. Thespaces are typically the same color as the background area, but may beof a different light color in this example. In other examples theelements of a bar code or matrix code are white or light colored and aredefined by black or darker colored spaces and background area.

In other applications, such as laser engraving on silicon wafers,illumination may result in a dark on light relationship in oneorientation and a light on dark relationship in a different orientation,In addition to pixel values representing reflective values of light("light" being defined as encompassing the entire electromagneticspectrum for present purposes), in other arrangements pixel valuesrepresentative of reflective values may be based upon reflection ofsound waves or other mediums from a dataform of an appropriateconfiguration. In any arrangement in which a dataform is arranged to beread on the basis of reflective values, such reflective values maytypically be stored as pixel values in an image buffer memory or otherstorage medium in bit map or other form which, while representative ofpixel values for an image, may utilize any appropriate data storageformat.

BACKGROUND OF SENSOR ARRAY READERS

As noted, prior arrangements for reading dataforms have been based uponlaser or continuous frame CCD scanners adapted for use withtwo-dimensional bar codes. However, these approaches have generally beensubject to one or more limitations in the quest for a practical, lowpower consumption, low cost, light weight hand-holdable reader providingfast and accurate reading of two-dimensional dataforms. For example, acontinuous frame reader typically consumes a full watt of power becausethe continuous frame architecture requires that the sensor arraycontinuously produces a stream of image data. When a microprocessordecodes a bar-code, it merely selects appropriate data to decode fromthe continuous stream of image data.

Full frame progressive scan CCD devices, as proposed for continuousframe transfer video or very high resolution still photograph capture,are subject to one or more of high cost, bulky configuration, high powerconsumption and slow gain control response time. Such factors limitapplicability to practical hand-held dataform reading applications.

Objects of the present invention are, therefore, to provide new andimproved dataform readers and methods avoiding one or more disadvantagesof prior arrangements.

Further objects are to provide dataform readers and methods capable ofproviding one or more of the following:

image capture with overlapping exposure periods for successive lines ofsensor elements;

automatic exposure control on a line-by-line basis;

overall low power consumption;

single frame image capture;

rapid automatic gain control;

automatic focus sensing and reading activation;

light weight hand-holdable configuration; and

single chip configuration capability.

SUMMARY OF THE INVENTION

In accordance with the invention, a dataform reader to read a dataformin a target area utilizes an array of sensor elements readable toprovide image signals, including a plurality of lines of sensorelements. An array control assembly is arranged to initiate chargeaccumulation by a line of sensor elements in response to an exposurestart signal and to cause image signals to be read from sensor elementsof the line in response to an exposure stop signal and coupled to anoutput point.

The dataform reader also includes an exposure control system coupled tothe output point and successively responsive to the level of imagesignals read from selected lines of sensor elements to determine anexposure period for at least one subsequent line of sensor elementsbased on the level of image signals from at least one of the selectedlines. The exposure control system is arranged to provide exposure startand stop signals to the array control assembly to implement suchexposure periods in a sequence causing the exposure period for one lineof sensor elements to overlap the exposure period for at least onesubsequent line of sensor elements.

Also in accordance with the invention, a method, for use with a dataformreader including an array of sensor elements, includes the followingsteps:

(a) causing illumination of a target area including a dataform to bereflected onto the array of sensor elements by turning on illuminationdevices;

(b) initiating charge accumulation by a first line of sensor elements inresponse to an exposure start signal initiating a first line exposureperiod;

(c) terminating the first line exposure period by reading image signalsfrom sensor elements of the first line, in response to an exposure stopsignal;

(d) coupling the first line image signals to an output point;

(e) repeating steps (b) through (c) for successive lines of sensorelements, with step (b) for each successive line of sensor elementsinitiated during the exposure period for the respective preceding lineof sensor elements, resulting in partially overlapping exposure periods;

(f) coupling image signals from successive lines of sensor elements tothe output point in sequence following signals from the respectivepreceding line; and

(g) terminating the reflection of illumination onto said array of sensorelements by turning off the illumination devices;

(h) coupling the image signals from the output point to a memory unit;and

(i) decoding the data form in a decoder coupled to the memory unit.

The described method may additionally include the following stepsbetween steps (d) and (e):

(x) determining an averaged level of first line image signals as coupledto the output point in step (d);

(y) utilizing the averaged image signal level to determine the timing ofan exposure stop signal applied in a repetition of step (c) to terminatethe exposure period of a line of sensor elements initiated after theexposure period for the first line.

For a better understanding of the invention, together with other andfurther objects, reference is made to the accompanying drawings and thescope of the invention will be pointed out in the accompanying claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A, 1B and 1C are respectively front, side and top views of ahand-held dataform reader utilizing the invention.

FIG. 2 is a block diagram of portions of the dataform reader with aconceptual side view of optical components of the reader.

FIG. 3 illustrates details of implementations of a portion of the FIG. 2system.

FIG. 4 is a conceptual side view illustrating aspects of an automaticfocus sensing system in accordance with the invention.

FIG. 5 is a block diagram showing a second embodiment of a portion ofthe FIG. 2 dataform reader.

FIG. 6 is an operational flow chart useful in describing operation ofthe illustrated dataform reader.

FIG. 7 is a flow chart useful in describing operation of the secondembodiment of the invention.

FIG. 8 illustrates, partially in block form, an embodiment of a dataformreader in accordance with the invention.

FIG. 9 is a flow chart useful in describing operation of the FIG. 8dataform reader.

DESCRIPTION OF THE INVENTION

An embodiment of a dataform reader utilizing the invention isillustrated in FIGS. 1A, B and C. FIG. 1A is a front conceptual view ofthe dataform reader 10 and FIGS. 1B and 1C are corresponding side andtop views, respectively. A portion of the upper casing is removed inFIG. 1B to provide a simplified view of internal components. Beforeaddressing specific aspects in accordance with the invention, it can beobserved that, as shown, the reader includes a suitable impact-resistantplastic case with a hand grip portion 12, a trigger device 14 andbattery compartment 16. The dataform reader also includes an upperenclosure portion 18 which, as illustrated in simplified form in FIG.1B, may include a sensor array assembly 20, illuminator array 22 andilluminator lens 24, each of which will be described in greater detail.FIG. 1B also depicts a processor and memory unit 30 and an input/output(I/O) unit 32, which may take the form of pluggable circuit boardsinserted into slots from the rear of the reader 10. Additional slots at34 and 36 may be utilized to provide additional or extended operatingcapabilities by enabling insertion of PCMCIA type cards, etc. As furtherdepicted in FIGS. 1B and 1C, the dataform reader 10 may include a dataentry keyboard 40 and a display 42, represented as adjustable todifferent viewing angles. These and other features may be provided byskilled persons using known techniques and types of components, exceptthat features and elements particularly relevant to implementation ofthe invention are provided as will be further described.

In the illustrated embodiment, there is provided an automatic exposuredataform reader 10 configured to read a dataform (such as atwo-dimensional bar code) existing in a target area positioned at adistance from the dataform reader. Thus, for example a bar code can beprinted on a label affixed to a package, component or letter and thedataform reader held by an operator, with the front of the reader at adistance from the bar code.

As shown in FIG. 1A, the reader 10 includes an array of illuminatorswith three different functions. Perimeter illuminators, such as shown at50, are positioned in a frame type configuration and arranged toilluminate the target area with a border or frame effect which indicatesthe field of view of the sensor array assembly 20. Focusingilluminators, shown at 52, are arranged in this embodiment to provideangled beams of light which intersect or overlap at a predetermineddistance in front of the reader. That distance represents a fixed focusdistance, as will be further described. Exposure illuminators, such asshown at 54 and which typically may be included in greater numbers, arearranged to provide a relatively uniform level of target areaillumination when turned on during an exposure period for the purpose ofreading the dataform (e.g., capturing an image of the dataform in thesensor array). Each of the illuminators may be an appropriate form ofdevice, such as a low cost light emitting diode (LED), arranged toprovide the respective levels of illumination determined to beappropriate in applications of the invention. The number, types,arrangement and utilization of the illuminators can be determined asappropriate. Depending upon the application, the perimeter illuminators50 or focusing illuminators 52 may be used alone or in combination toprovide exposure illumination during exposure periods. The illuminatorlens 24 may comprise an array configuration including a small lensportion in front of each of the illuminators 50, 52 and 54 in order toprovide appropriately focused beam configurations for each of therespective functions already discussed. In the FIG. 1A view a centrallens 56 is arranged to focus upon the face of the sensor array containedin assembly 20 illumination reflected from the target area and anyincluded dataform, in order to enable the array to sense the image andprovide image signals.

Referring now to FIG. 2, there is shown a simplified block diagram ofportions of the dataform reader utilizing the invention. A conceptualcross-sectional view of related optical elements is included. As shown,sensor array assembly 20 projects through lens assembly 24 and the arrayof illuminators 50 and 54 and includes a sensor array 21, optical filter26 and array control unit 28, with associated clock device 29. Sensorarray 21 is positioned behind (to the right of, in this side view)central lens 56 and filter 26. By providing a filter 26 which istransmissive to illumination provided by the illuminators, but effectiveto reduce transmission in other portions of the ambient light spectrum,the effects produced by ambient light during the exposure period arereduced.

As noted, sensor array assembly 20 may include a two-dimensional array21 of sensing cells (each utilizing a photodiode and responsive toincident reflected light). Array control unit 28 may typically includevertical and horizontal readout circuits, devices for sensing charge orvoltage appearing at individual sensing cells, and an output amplifierdevice with adjustable gain for coupling image signals from the sensorarray assembly 20, as well as clock device 29 for providing timingcontrol of the reading of image signals from selected sensor elements.An arrangement of this type, suitable for use in the dataform reader 10,is shown and described in U.S. patent application Ser. No. 08/258,428filed Jun. 10, 1994, and having a common assignee herewith. The contentof such copending application is hereby incorporated by reference. Whileother types of sensor array arrangements may be utilized inimplementation of the invention, an advantage in use of the type ofarrangement of the referenced patent application is that the entiresensor array, plus some or all of the associated gain control, focussensing and exposure control circuitry, may be enabled to be implementedon a single chip using known application of CMOS technology (or PMOS,NMOS, Bipolar, BiMOS, BiCMOS, or other existing or newly availabletechnology). Use of existing CMOS technology, for example, is effectiveto provide significant advantages of established production techniques,single chip size, weight and cost advantages and, possibly mostimportantly, low power consumption (as compared to higher powerrequirements of prior CCD or other arrangements whereby supportcircuitry for the sensor array is located off chip).

In FIG. 2, the sensor array is focused, via lens 56, on target area 58which is at a distance 59 from lens 56. The filter, 26 is placed betweenthe lens 56 and the sensor array. Filter 26 can be specified so that itis primarily transmissive only to light in a particular portion or bandof the electromagnetic spectrum and is effective to reduce transmissionin other portions of the ambient light spectrum (e.g., portionsdiffering from the transmissive band or portion). With this approach,the sensor array can be arranged to be relatively non-responsive toambient light reflected from the target area.

FIG. 2 also includes an in-focus sensing device 62 responsive to imagesignals provided from a plurality of sensor elements and arranged toprovide an "in-focus" signal usable to initiate a dataform readingsession. The in-focus signal is provided when an area of illuminationprovided by the focus illuminator or illuminators is characterized byhaving at least one of (a) a size within a predetermined size range, (b)a brightness within a predetermined brightness range, and (c) a locationwithin a predetermined location range, as represented by such imagesignals. FIG. 3 indicates two arrangements for providing appropriateimage signals to device 62. In FIG. 3, 56 represents an outline of thearray focusing lens and 21a represents the outline of an array ofsensing elements included in sensor array assembly 20. At 21b isindicated a linear sensor which may comprise one or two lines of sensorelements provided separately from the element array 21a. Linear array21b is connected to point 48 of FIG. 2 under the control of arraycontrol unit 28 (not shown in the simplified representation of FIG. 3).

FIG. 4 is a representation of focus illuminators 52 providing, via lensassembly 24, angled light beams 52a and 52b as previously discussed. Asshown, these beams intersect or cross at a distance 59 from the front ofthe lens 56. At distance 59, there is represented a side view of theplane of focus 70 of the sensor array of array assembly 20 incombination with focusing lens 56 (see also FIG. 2).

Thus, with particular choices of a sensor array configuration and lens,the dataform reader will exhibit an in-focus condition, with an image ofthe target area and any included dataform accurately focused on thesensor elements of array 21, if the target area lies in the plane 70which is at a distance 59. Further, the lens 56 can be specified so asto provide a reasonable depth of focus, with the result that an image ofthe target area will be acceptably focused on the sensor elements forany separation distance within the depth of focus range indicated at 72.Once the distance 59 has been determined for a particular reader design,the beam angles of illuminators 52 can be adjusted to provide beamintersection or overlap at the distance 59, as shown in FIG. 4. Withthis arrangement, linear sensor 21b of FIG. 3 will initially provideimage signals representative of target area illumination by two spots oflight located at spaced positions when the target area lies in plane 74at a distance 76. Then, as the dataform reader is moved closer to thetarget area so that the target area lies in plane 70 at distance 59, thetwo spots of light will converge into a single brighter spot at acentral location.

The image signals from linear array 21b will thus provide informationrepresentative of the single brighter spot of illumination and itslocation, thereby providing information indicative of the in-focuscondition. By providing a degree of tolerance on the in-focus imagesignal indication, the in-focus indication can be adjusted toaccommodate the depth of focus range 72. Upon successful distanceadjustment (e.g., user movement of a hand-held reader closer or fartherfrom the dataform image) to achieve an in-focus indication, in-focussensing device 62 is arranged to provide an "in-focus" signal usable forinitiating a reading and decoding cycle. It will be apparent that thearrangement as described also enables operation in a manual in-focusdetermination mode. Thus, with the operator adjusting the position ofthe dataform reader relative to the target area and observing theconvergence of the two spots of light into a single spot, as described,an in-focus indication can be provided by operator activation of anappropriate key of keyboard 40 when convergence is achieved.

With reference to FIG. 3, the dotted connection between sensing elementarray 21a of assembly 20 and circuit point 48 indicates an alternativeconfiguration. As shown in FIG. 2, point 48 provides connection toin-focus unit 62 (as well as units 60 and 64). Instead of providingadditional sensing elements necessary in order to provide a dedicatedlinear sensor 21b, it can be arranged to make temporary use of one ormore lines of elements of array 21a for focusing purposes, independentlyof the basic image sensing function of array 21a. With the latterarrangement, the desired in-focus determination can thus be made withoutthe requirement to provide any separate linear sensor such as 21b.

As shown, FIG. 2 further includes an exposure control device 64responsive to image signals from one or more selected sensor elementsand arranged to provide "start" and "stop" signals usable for beginningand terminating an exposure period. Exposure control 64 utilizes theimage signals to provide the stop signal in response to reflection of apredetermined level or intensity of illumination. Typically, suchpredetermined level or intensity will be measured within a periodinitiated by the start signal provided by the exposure control deviceand may represent an accumulated representation of the intensity ofreflected light over time. By converting image signals received in theperiod to a voltage representative of accumulated image signal levels,and comparing that voltage to a preset threshold voltage, the stopsignal can be generated when the accumulated voltage reaches thethreshold voltage, representing a predetermined illumination exposure ofthe target area.

In another embodiment illustrated in FIG. 5, the exposure control devicesets the duration of the time between the start and stop signals byresponding to the illumination intensity as measured by a preset fixedtime period sample exposure of one or more selected sensor elements. Theimage signals from such sensor elements (typically, two lines of sensorelements, as discussed above) will thus be representative of the levelof illumination reflected from the target area during the preset sampleexposure period. By converting the image signals to a gray level signal,an exposure control signal representative of the appropriate duration ofthe adjustable exposure period is provided. In order to determine theactual duration of the exposure period represented by the controlsignal, the exposure control device 64a is coupled to the CPU 88. Asshown in FIG. 5, the CPU is arranged to access a look-up table (storedin memory unit 82a) containing exposure period data correlated to graylevel signal values. The actual look-up table data can be derived inadvance on an empirical or other appropriate basis utilizing the levelof reflected light during the preset initial period of predeterminedduration as an indication of the exposure time which will be required toenable the capture of usable image data on a single frame activationbasis.

As also indicated in FIG. 2, gain control device 60 is arranged torespond to image signals provided from one or more of the sensorelements of array assembly 20, and more particularly to the level ofreflected light represented by such image signals, to control imagesignal amplification. The gain control in this embodiment is achieved bya gain control signal coupled back to the above-referenced adjustablegain output amplifier included in the sensor control unit 28. Thisenables the amplitude of the image signals provided by the sensor arrayto be maintained within a predetermined range substantiallyindependently of reflected ambient illumination as represented byamplitude levels of selected image signals.

As illustrated in FIG. 2, this embodiment of the dataform reader inaccordance with the invention also comprises a processing unit 80,memory unit 82 and input/output (I/O) module 84. Processing unit 80,which may include a digitizer 86, CPU 88 and power management module 90,receives image signals from sensor array assembly 20 and provides imagedata in digitized form for storage in memory unit 82. Unit 80 isresponsive to the start and stop signals from units 62 and 64 to controlthe exposure period. As will be further described, during the operatingsequence processing unit 80 is also arranged, via power managementmodule 90 coupled to a battery (not shown), to turn on and off theperimeter, focus illuminators 50 and 52, and exposure illuminators andcouple power for operation of the sensor array assembly 20. Processingunit 80 is further arranged to implement decoding of a dataform usingimage data stored in memory unit 82. Upon successful decoding of thedataform, unit 80 also provides an end-cycle signal effective toterminate decoding operation and also to end the reading of sensorelements to provide image signals, by terminating at least one of thecoupling of input power and provision of clock signals which are bothrequired in the reading of sensor elements under the control of arraycontrol unit 28.

Separately, decoded dataform information is provided to an output device92, via I/O module 84. The I/O module 84 may be arranged to operate withPCMCIA cards in interface slots 34 and 36 discussed with reference toFIG. 1B, and may be arranged to provide radio, infrared, wired or othersignal transmission and reception capabilities. Output device 92 mayaccordingly be an output port for coupling output signals via aconductor, an antenna or optical device for radio or infraredtransmission, or other suitable device, with I/O unit 84 arranged toprovide the decoded dataform information in suitable form for use withthe particular form of output device. Modem, speech recognition,handwriting recognition, memory and other types of additional capabilityor peripheral cards may also be inserted in the PCMCIA slots foroperation in cooperation with processing unit 80 and I/O module 84 toprovide extended and further features. Items not specifically describedcan be provided by persons skilled in the relevant technologies.

With an understanding of the dataform reader as described, it will beapparent that for dataform reading and in other applications an imagingsystem provided in accordance with the invention may include automaticgain control, automatic exposure, automatic focus sensing, single frameimage capture and other features as described.

OPERATION

With reference now to FIG. 6 there is shown an operational flow chartwith reference to operation of a dataform reader utilizing theinvention. At step 100, a user activates trigger device 14 of dataformreader 10 shown in FIG. 1B. At step 104, perimeter illuminators 50 andfocus illuminators 52 are turned on and reading of sensor elements isinitiated. At step 106, the user adjusts the distance between thedataform reader 10 and the target area to achieve a separation distancewithin range 72 in FIG. 4, at which point the areas of illuminationintersect and merge into a single smaller, brighter area or spot ofillumination having a central location. At step 108, the focus conditionachieved in step 106 is monitored on the basis of image signals from alinear array of sensors indicative of whether the area of illuminationis characterized by at least one of (a) a size within a predeterminedsize range, (b) a brightness within a predetermined brightness range,and (c) a location within a predetermined location range, or anycombination of the three, as will occur as the two illumination areas,as provided on the target area by beams 52a and 52b in FIG. 4, overlapand merge. For two round spots of illumination, the spots will thusbecome concentric when focused and this minimum size condition can bedetected in a variety of ways, including detecting the relativepositions of the two spots within the field of view. When suchillumination area merge is achieved as characterized, an "in-focus"signal is effective at step 110 to turn on all illuminators of theexposure array (e.g., illuminators 50 or illuminators 50 and 52,depending upon the particular configuration). As discussed, suchin-focus signal can be implemented automatically or manually based onoperator observation.

Upon turning on the exposure illuminators, the exposure control devicesends a start signal to sensor array assembly 20 which is effective toreset any accumulated charge on the sensors to a reference charge. Thephoto sensors immediately begin accumulating a new charge as indicatedat step 112. Simultaneously the exposure control device and the gaincontrol device periodically measure accumulated charge on a sample ofphotodetectors at steps 113 and 114. The gain control device at step 113uses sample image data to select an appropriate amplitude gain andoffset signal to apply to the sensor array amplifier in array controlunit 28. At step 114, the exposure control device monitors the sampleimage data and when the sample image data indicates that the level ofreflected light from the target area, on a cumulative basis, has reacheda predetermined level, the exposure control device generates a stopsignal. In response to the stop signal the accumulated charge on theexposed sensor is measured and converted to a voltage signal. Knowntypes of sensor arrays utilizing two-dimensional arrays ofphotosensitive cells are structured so that sensor elements are groundedto a reference charge level and then permitted to accumulate chargeduring an exposure period. Then, pursuant to a reading process, eitherall or selected cells (e.g., one half of the cells, in an interlacedconfiguration, or one line in a line-byline readout arrangement) aresampled simultaneously to measure accumulated charge, with datatemporarily stored and read out line-by-line sequentially using a shiftregister arrangement. At step 115, if no more cells require readout(e.g., all cells have been sampled simultaneously) the exposureilluminators are turned off. However, if the configuration is such thatadditional cells remain to be read, in this embodiment the system willreturn to steps 112 and 113. The exposure control device will thengenerate a start signal to initiate an exposure period for the nextgroup of cells, which will be read out at the end of that exposureperiod. After reading a complete frame, the system will advance fromstep 115 to step 116 at which point the exposure illuminators are turnedoff.

At step 117, processor unit 80 attempts to decode the dataform utilizingimage data consisting of image signals from array assembly 20 which havebeen digitized and stored in memory unit 82. If decoding is successful,at step 118 the decoded dataform information is made available fortransmission out of the dataform reader 10 and an end-cycle signal isprovided to terminate the reading cycle by turning off at least one ofinput power and clock signals as utilized by the array control unit 28.If the decoding is not successful, at step 117 the reading cycle isreactivated or repeated starting at step 104, as indicated in FIG. 6.

It should be noted that in step 117, if a dataform is in fact present inthe captured image of the target area, it will typically be necessary tolocate the dataform within the image to enable decoding. Location of thedataform image can be accomplished as described in U.S. Pat. No.5,304,787, entitled "LOCATING 2-D BAR CODES", issued Apr. 19, 1994, andhaving a common assignee.

Consistent with the foregoing, a method, for use with a dataform readerincluding an array of sensing elements, includes all or selected ones ofthe following steps, particularly in application of the invention to thereading of a dataform:

(a) positioning in front of the array an optical filter transmissive tolight from an exposure illuminator (described below) and effective toreduce transmission in other portions of the ambient light spectrum;

(b) initiating reading of selected sensor elements by providing inputpower and clock signals required for such reading;

(c) illuminating a target area with an area of illuminationcharacterized by at least one of a size, a brightness and location whichvaries with the distance between the array and the target area;

(d) adjusting such distance to cause the area of illumination to becharacterized by at least one of a size within a predetermined sizerange and a brightness within a predetermined brightness range and alocation within a predetermined location range;

(e) providing an in-focus signal when an image signal from at least onesensing element indicates that the illumination is characterized asdescribed in step (d);

(f) turning on an exposure illuminator in response to the in-focussignal;

(g) utilizing image signals from selected sensing elements, asrepresentative of the level of reflected light, to provide a gaincontrol signal to control the amplification of image signals from thearray;

(h) providing a stop signal when image signals from at least one sensingelement indicate reflection of a predetermined level of illuminationfrom the target area;

(i) upon complete exposure of the sensor cells, turning off the exposureilluminator; and

(j) processing image data, representing image signals from the arraywhich have been digitized and stored in memory, to decode the dataform;

(k) upon successful decoding of the dataform, providing an end-cyclesignal, ending sensor reading by terminating at least one of the inputpower and clock signals, and coupling decoded dataform information to anI/O module; and

(l) if decoding is unsuccessful, repeating the method from step (d).

FIG. 7 shows a flowchart corresponding to the second embodiment of thisinvention. Steps 100 to 108 operate the same as described in theprevious embodiment. After determining the in-focus condition at 108 andgenerating an in-focus signal, the exposure illuminators are turned onfor a preset sample exposure period and image data is collected at step122. To do this, the exposure control device generates a sample exposurestart signal whereby selected photo sensors are grounded to a referencecharge and begin accumulating a sample charge. At the end of the presetexposure period, the exposure control device, a portion of which couldsimply be a timer for this purpose, generates a stop signal whereby thesample accumulated charge on each selected sensor is read as image data.At 124, in response to image data collected during the sample exposure,the exposure control device determines the appropriate duration of anadjustable exposure period. As discussed, the appropriate duration ofthe exposure period may be determined by accumulating, via exposure unit64a of FIG. 5, image data from the selected sensors and referring aresulting voltage level to a look up table stored in memory 82a.

It will be appreciated that the level of reflected illumination will bedetermined by, among other possible factors, the reflectance of thetarget area. Such reflectance may be substantially higher or lower thanan expected or typical value in certain conditions of surface texture orcoloration. Accordingly, it may be desirable to control the gain ofimage signals from the array, as well as the exposure period. Thisresult can be provided by accumulating, via gain unit 60a of FIG. 5,image data from selected sensor elements and referring a signalrepresentative thereof to a look-up table in memory 82a which, forparticular levels of illumination reflected during the preset initialperiod, provides values for adjustment of image signal output gain. Withan understanding of the invention, skilled persons will be enabled toprovide appropriate look-up tables utilizing empirical or othertechniques. FIG. 7 thus provides step 126 for using the sample imagedata to determine an appropriate gain adjustment to apply to the outputamplifier of the sensor array assembly.

At 128 the device captures a single frame of image data. As discussedabove, if the sensor array is structured so that all photosensor cellsare referenced, exposed, and sampled in parallel to generate a fullframe of data, then step 128 will consist of only one cycle ofgrounding, exposing and sampling the accumulated charge on the cells. Atstep 130 the exposure illuminator is turned off. As further discussed,if the photosensor array is structured so that only selected sensorelements may be read in parallel in a single cycle, the exposure controldevice will generate a plurality of start and stop signals correspondingto the predetermined exposure time as indicated by dashed path 128, asappropriate to complete the reading of all cells. After collecting afull frame of data, at 130 the exposure illuminators are turned off. Ifthe data form is successfully decoded at 132 the data transmission andtermination of the reading cycle, including termination of at least oneof the input power and clock signals utilized by the array controldevice, proceed at step 134.

Simplicity and efficiency of operation are enhanced by automatic gaincontrol, automatic no-shutter exposure control and automatic in-focussensing on a hand-held, user positioned basis. Operative advantagesinclude full resolution, full frame image capture on a single frame,automatic exposure (e.g., shutter speed) basis regardless of ambientlight levels. Necessary gain adjustment can be sensed in a period of theorder of 100 microseconds. With single frame image capture, continuousimage data transfer and data processing is avoided. In addition tohand-held applications, the simplicity, cost and reliability advantagesof imaging systems in accordance with the invention are readily adaptedfor use in automated, fixed-position, non-attended applications fordataform reading and other imaging purposes. Additionally, the inventionprovides the advantage that, using available CMOS or other technology,the sensor array assembly 20 and all or major portions of units 60, 62,64 and 80 can be fabricated on a single chip enhancing small size, lightweight, ease of packaging and low power consumption (e.g., as low asone-tenth the power consumption of comparable CCD array components).This enables provision of a small, lightweight, long operating periodhand-held battery operated unit suitable for reading dataforms or otherimage capture applications.

LINE EXPOSURE ARRANGEMENTS OF FIGS. 8 AND 9

Prior sensor array scanners focus an image of a target area onto asensor array to simultaneously expose all or half of the sensorelements. Thus, for example, image capture may be accomplished bysimultaneous exposure of all alternate lines of a sensor array followedby sequential read out on a line-by-line basis. This interlaced exposureapproach is then completed by simultaneously exposing the remaininglines in a second exposure period. In each step there is one exposureperiod, which subjects the image capture process to a number ofpotential problems or shortcomings. Non-uniform image illumination orsurface reflectivity can result in inclusion of image areas which areoverexposed or underexposed when an exposure period is determined forthe average illumination associated with an entire image. Also, theexposure period may be initiated at a given time and then continue foreach line until the exposure time for that line is terminated by theimage data being read out of the line of sensor elements. With thisapproach, as the lines are read out in sequence, the exposure time forthe last line is significantly longer than for the first line. As aresult the first line may be underexposed and the last line overexposedto the point of loss of image data.

Referring now to FIG. 8, there is shown an embodiment of a dataformreader including automatic line exposure features usable in the FIG. 2dataform reader in accordance with the invention. As illustrated, theFIG. 8 dataform reader includes an array 21c of sensor elements,indicated typically at 140, which are readable to provide image signals.As shown, the sensor elements 140 are arranged in a plurality ofhorizontal lines and vertical columns.

The FIG. 8 dataform reader also includes an array control assemblycorresponding generally to array control unit 28 of FIG. 2. As shown,the FIG. 8 array control assembly comprises a line control shiftregister 142 which is coupled to successive lines of the sensor elements140 by way of common horizontal conductors 143 connected to flip-flopunits 144-151. Vertical columns of sensor elements 140 are alsoconnected to common vertical conductors 152, which connect to groundingswitches shown typically at 154 and to sample and hold amplifiers156-162, via sensor readout switches shown typically at 168. The FIG. 8dataform reader further includes output shift register 166 arranged tosuccessively activate the sensor readout switches, shown typically at168, individually connected to the flip-flop units 170-176, to causeimage data stored in sample and hold amplifiers 156-162 to besuccessively coupled to output point 178 for successive sensor elements140 of a selected line of the array 21c.

As illustrated, the FIG. 8 dataform reader includes an exposure controlsystem 64/80, generally corresponding to the combination of exposurecontrol device 64 and processing unit 80 of FIG. 2, adapted to operateas will be described with reference to FIG. 8. As shown in FIG. 8,exposure control system 64/80 is arranged to control: sensor elementline selection via shift register 142; reference potential grounding ofcolumns of sensor elements via control of switches 164; readout of imagesignals from a selected line of sensor elements via switches 156-162;and output of image signals from successive sensor elements of aselected line by control of shift register 166, to control activation ofsensor readout switches 168 to couple image signals to output point 178.Output point 178 may typically be coupled to the input of digitizer 86of FIG. 2, enabling the image data to be stored in memory 82 fordecoding, transmission or other use. Exposure control system 64/80 isalso coupled to output point 178, via conductor 180, so as to enable anaveraged image signal level to be obtained from a particular line of thesensor elements 140. Such image signal level is thus made available foruse in determining the exposure time for a subsequent line or subset oflines of sensor elements. Variations which will be apparent to skilledpersons for use in various applications of the invention includeobtaining such an averaged image signal level from more than one line ofsensor elements (e.g., from elements of two successive lines). Basicconstruction and operation of the sensor array and related portions ofthe FIG. 8 dataform reader are as described in the above-referencedcopending application Ser. No. 08/258,428 or familiar to skilledpersons, except to the extent of aspects unique to the presentinvention, which will be described in further detail.

Operation of the FIG. 8 dataform reader can be more fully described withreference to the FIG. 9 flow chart. At step 200 operation is initiatedby a user activated trigger signal in order to read a dataform in theform of a bar code printed within a target area on a surface, forexample. At 210 exposure illuminators, such as shown at 54 in FIG. 2,are turned on to illuminate the bar code within the target area.

At step 220A, exposure control system 64/80 of FIG. 8, acting by way ofcontrol signals provided to shift register 142, flip-flop 144 andgrounding switches 154, couples each of the sensor elements in the firsthorizontal line of the elements 140 to a reference potential (e.g., toground) in order to remove any accumulated charge on the first line ofelements (e.g., photo cells). In this manner, the signal from exposurecontrol system 64/80 which is effective to close the grounding switches154 acts as an exposure start signal for the first line of sensorelements 140 which are connected to flip-flop 144. Thus, as illuminationprovided at step 210 is reflected onto the array of sensor elements, theexposure period for the first line of elements begins immediately afterthose elements are set to reference potential at step 220A and thenbegin to accumulate charge representative of the level of illuminationreflected from different portions of the bar code in the target area.

At step 222A charge is accumulated on the sensor elements of the firstline in an exposure period. This initial exposure period for the firstline may be determined in any appropriate averaged, standardized orother manner to initiate the image data readout process. At step 224A anexposure stop signal is provided to the readout switches 164, viaconductor 163, to cause each one of the sensor elements in the firstline connected to flip-flop unit 144 to be coupled to a respective oneof the sample and hold amplifiers 156-162. Image data is therebysimultaneously read out of all of the sensor elements of the first line,with the image data from each sensor element respectively held in one ofthe amplifiers 156-162. The exposure period for each sensor element ofthe first line is thus initiated by the exposure start signal causingthe elements to be grounded and simultaneously terminated when exposurestop signal causes the elements to be read into the sample and holdamplifiers.

At step 226A shift register 166 is activated to control flip-flop units170-176 which sequentially actuate the switches 168 to cause a signalcomprising a sequence of data representative of the level of imagesignals read from the individual sensor elements of the first line to becoupled to output point 178.

The same series of steps, denoted at 220B, 222B, 224B and 226B in FIG.9, are carried out for the second line of sensor elements. Asrepresented in FIG. 9, because the exposure period required toappropriately expose a line of cells is longer than the time required tosample the cells to read the image data and output the data to outputpoint 178, the controlled exposure period for the next line of sensorelements is initiated during the preceding exposure period. Thus, withthe passage of time represented in a downward direction in FIG. 9, itwill be seen that the exposure periods 222A, 222B and 222C for the firstthree lines of sensor elements begin on a successive time-steppedsequence so that the exposure period 222B overlaps the exposure period222A for the preceding line of sensor elements in this example. FIG. 9is provided for illustrative purposes and actual timing relationshipsmay be different than those shown. The timing is controlled so that thefirst, second and third lines are read and the resulting image coupledto output 178 in succession, so as to produce a stream of image data. Inthis way, image data for each successive sensor element of the secondline follows the corresponding image data for the first line, etc.,without interference. This is accomplished under the control of asequence of exposure periods determined by exposure start and stopsignals provided for each line of sensor elements, respectively by theexposure control system 64/80. The sequence of overlapping timecontrolled exposure periods for successive lines continues until alllines have been exposed, read and image data coupled to output point178, as represented by corresponding steps 220N, 222N, etc. implementedfor the last, or bottom line, denoted line N.

At step 228 the exposure illumination is turned off. At 230 the dataformreader checks for a successful decoding of the bar code present in thetarget area in this example. If the bar code has been successfullydecoded the decoded data is provided, at step 232, to the FIG. 2 outputport 92. If the decoding is found to have been unsuccessful in providingthe level of decoded data desired, at step 230 the operation is returnedto step 210 and the illumination is turned on for an iteration of theprocess.

With an understanding of the basic exposure and reading process,attention is again directed to the dataform reader as shown in FIG. 8.Assume the first line of sensor elements has been exposed, read and theresulting image data coupled to output point 178 for coupling todigitizer 86 of FIG. 2 for further processing and use. As shown, arepresentation of the first line image data is also coupled to theexposure control system 64/80, via conductor 180. By monitoring thelevel of such image data representative of the first line, anappropriate exposure period can be determined for a subsequent line andused for determining the exposure start and stop periods for thatsubsequent line. For example, since the FIG. 9 illustration shows thethird line exposure period 222C beginning after the step 226A outputfrom the first line, exposure information derived from the first lineimage signal level can be used for controlling the exposure period usedfor the third line. Alternatively, instead of independently determiningthe exposure period for each successive line individually, the exposureinformation derived from the first line image signal level can be usedfor controlling the exposure period for a subsequent subset of lines,for example the third, fourth and fifth lines. This line subset approachprovides exposure period accuracy between that provided by a single fullimage (or interleaved image) exposure period and the higher accuracypossible by use of an exposure period determined for each subsequentline individually. Development of the exposure information can becarried out by averaging the level of image data for a plurality ofsensor elements of a given line and utilizing a look-up tablearrangement, as discussed with reference to FIG. 5, to determine theactual duration of an appropriate exposure period for a subsequent line,or subset of lines, of sensor elements. In utilizing such an exposureperiod for exposure of a subset of three lines, for example, each linecan be grounded in series and read out in series successively, using thesame exposure period as determined from the level of the first lineimage data.

Addressing the operation of the FIG. 8 arrangement more particularly,exposure control system 64/80 is effective to send control signals toline control shift register 142 to determine which line (e.g., thehorizontal line of sensor elements coupled to flip-flop unit 144) shouldhave the voltage driven high and which lines (e.g., all remaining linesof sensor elements) should have the voltage driven low. Under control ofthe shift register 142, the respective flip-flop units 144-151 drive thesensor element line voltages appropriately. When a line is driven high,the charge accumulated on each sensor element in that line istransferred to the respective one of the vertical conductors 152connected to that sensor element. When a horizontal line is driven high,the exposure control system 64/80 also sends a signal to either thegrounding switches 154 or the readout switches 164. In the first case,the charge transferred to each respective vertical conductor 152 isgrounded. In the latter case, each sensor element of the selected lineis read by having charge from it coupled to the respective one of thesample and hold amplifiers 156-162, so that a representation of imagedata on each sensor element is saved in the amplifiers 156-162 as avoltage level for later output coupling.

At the beginning of an exposure period a selected line is driven highand the grounding switches 154 are closed in response to an exposurestart signal provided via conductor 155, in order to initiate a lineexposure period. At the end of the exposure period, the line is drivenhigh and the readout switches 164 to the sample and hold amplifiers156-162 are closed by an exposure stop signal provided via conductor163. After a line of image data is collected in the sample and holdamplifiers, the exposure control system 64/80 causes the image data readfrom each sensor element to be sequentially coupled to the output point178. This is accomplished by control signals sent to readout shiftregister 166, which controls flip-flop units 170-176. The flip-flopunits each connect to one of the output switches 168, so that the imagedata held in each of amplifiers 156-162 can be coupled sequentially tooutput point 178. In this manner, with appropriate timing, image datafrom each sensor element in a horizontal line and from successive linesof elements can be coupled to output point 178 on a continuoussequential basis to provide image data representative of a completeframe. From output point 178 the image data is provided for furtherprocessing as previously discussed and also provided as an input toexposure control system 64/80 for use in determining exposure periodsand providing appropriate exposure start and stop signals.

Pursuant to the foregoing, a method, for use with a dataform readerincluding X lines of sensor elements arranged in an array, may desirablyinclude the following steps:

(a) reading image signals from sensor elements of a selected line ofsensor elements;

(b) utilizing the level of image signals read in step (a) to determinean exposure period;

(c) utilizing such exposure period to control the duration of exposureto illumination reflected from a target area onto a first subset of Ylines of sensor elements, where Y is at least one and less than X; and

(d) repeating steps (a) through (c) substituting in step (a) imagesignals from a line subsequent to said selected line, and in step (c)utilizing the exposure period to control the duration of exposure of asecond subset of Y lines subsequent to the first subset of Y lines.

For example, with Y equal to three, each line of sensor elements of asubset of three lines can be exposed for an identical exposure perioddetermined on the basis of the level of image signals read from anearlier selected line of sensor elements. Also, where Y equals one, eachsubset of one line of sensor elements can be exposed for an exposureperiod determined on the basis of the level of image signals read froman earlier selected line of sensor elements, so that the exposure periodfor each line is independently determined.

While there have been described the currently preferred embodiments ofthe invention, those skilled in the art will recognize that other andfurther modifications may be made without departing from the inventionand it is intended to claim all modifications and variations as fallwithin the scope of the invention.

What is claimed is:
 1. A dataform reader comprising:an array of sensorelements readable to provide image signals, including a plurality oflines of sensor elements; an array control assembly coupled to saidarray and arranged to initiate charge accumulation by a line of sensorelements in response to an exposure start signal, and to cause imagesignals to be read from sensor elements of said line in response to anexposure stop signal and coupled to an output point; and an exposurecontrol system coupled to said output point and successively responsiveto the level of image signals read from selected lines of sensorelements to determine an exposure period for at least one subsequentline of sensor elements based on the level of image signals from atleast one of said selected lines, and arranged to provide said exposurestart and stop signals to said array control assembly to implement saidexposure periods in a sequence causing the exposure period for one lineof sensor elements to overlap the exposure period for at least onesubsequent line of sensor elements.
 2. A dataform reader as in claim 1,wherein said array control assembly is arranged to cause individualimage signals from complete lines of sensor elements to be sequentiallyread and coupled to said output point in response to an exposure stopsignal.
 3. A dataform reader as in claim 1, wherein said exposurecontrol system is arranged to utilize the image signals from a pluralityof sensor elements of a selected line of said array to develop anaverage image signal level which is usable for determining an exposureperiod for one of the following: a single subsequent line of sensorelements, a plurality of successive lines of sensor elements.
 4. Adataform reader as in claim 3, additionally comprising a memory unitstoring a look-up table of line exposure periods versus average imagesignal levels.
 5. A dataform reader as in claim 1, wherein said arraycontrol assembly, upon initiation of charge accumulation, causes a lineof sensor elements to be set to a reference potential by being coupledto a ground reference potential.
 6. A dataform reader as in claim 1,wherein said exposure control system provides said start signals to saidarray control unit for successive lines of sensor elements atpredetermined uniform time intervals.
 7. A dataform reader as in claim1, wherein said array control assembly comprises a line control shiftregister unit enabling individual element sensor lines of said array tobe selected for one of reading and coupling to said reference potential,and a readout shift register unit enabling individual element sensors tobe selected for coupling of image values to said output point.
 8. Adataform reader as in claim 1, additionally comprising:a memory unitcoupled to said output point and arranged to store image datarepresentative of said image signals; and a processing unit coupled tosaid memory unit and arranged to process said stored image data todecode a dataform represented by said image signals.
 9. A dataformreader comprising:an array of sensor elements readable to provide imagesignals, including a plurality of lines of sensor elements; an arraycontrol assembly arranged to initiate, in response to an exposure startsignal, charge accumulation by a line of sensor elements set to areference potential and to cause image signals to be read from sensorelements of said line in response to an exposure stop signal and coupledto an output point; and an exposure control system coupled to saidoutput point and successively responsive to the level of image signalsread from selected lines of sensor elements to determine an exposureperiod for at least one subsequent line of sensor elements based on thelevel of image signals from at least one of said selected lines, andarranged to provide said exposure start and stop signals to said arraycontrol assembly to implement said exposure periods for lines of sensorelements.
 10. A dataform reader as in claim 9, wherein said exposurecontrol system is arranged to utilize the image signals from a pluralityof sensor elements of a selected line of said array to develop anaverage image signal level which is usable for determining an exposureperiod for one of the following: a single subsequent line of sensorelements, a plurality of successive lines of sensor elements.
 11. Adataform reader as in claim 10, additionally comprising a memory unitstoring a look-up table of line exposure periods versus average imagesignal levels.
 12. A dataform reader as in claim 9, wherein saidexposure control system provides said start signals to said arraycontrol unit for successive lines of sensor elements at predetermineduniform time intervals.
 13. A dataform reader, usable to read a dataformin a target area at a distance from said reader comprising:an array ofsensor elements readable to provide image signals, including a pluralityof lines of sensor elements; at least one exposure illuminator arrangedto illuminate said target area; a focusing device positioned in front ofsaid array and arranged to focus on said array illumination reflectedfrom at least a portion of said dataform when said distance is within afocus range; an array control assembly coupled to said array andarranged to initiate charge accumulation by a line of sensor elements inresponse to an exposure start signal and to cause image signals to beread from sensor elements of said line in response to an exposure stopsignal and coupled to an output point; an exposure control systemcoupled to said array control assembly and arranged to provide saidexposure start and stop signals to said array control assembly toimplement said exposure periods in a sequence causing the exposureperiod for one line of sensor elements to overlap the exposure periodfor at least one subsequent line of sensor elements; and a processingunit coupled to said output point and arranged to process said imagesignals to decode said dataform.
 14. A dataform reader as in claim 13,wherein said exposure control system provides said start signals to saidarray control unit for successive lines of sensor elements atpredetermined uniform time intervals.
 15. A method, for use with adataform reader including an array of sensor elements, comprising thefollowing steps:(a) causing illumination of a target area including adataform image to be reflected onto said array of sensor elements; (b)initiating charge accumulation by a first line of sensor elements inresponse to an exposure start signal initiating a first line exposureperiod; (c) terminating said first line exposure period by reading imagesignals from sensor elements of said first line in response to anexposure stop signal; (d) coupling said first line image signals to anoutput point; (e) repeating steps (b) through (c) for successive linesof sensor elements, with step (b) for each successive line of sensorelements initiated during the exposure period for the respectivepreceding line of sensor elements, resulting in partially overlappingexposure periods; (f) coupling image signals from successive lines ofsensor elements to said output point in sequence following image signalsfrom the respective preceding line; and (g) terminating said reflectionof illumination onto said array of sensor elements.
 16. A method as inclaim 15, wherein step (a) includes turning on illumination of saidtarget area to provide reflected illumination and step (g) includesturning off said illumination after image signals have been read fromall lines of sensor elements of said array.
 17. A method as in claim 15,additionally including the following steps between steps (d) and (e):(x)determining an averaged level of first line image signals as coupled tosaid output point in step (d); (y) utilizing said averaged image signallevel to determine the timing of an exposure stop signal applied in arepetition of step (c) to terminate the exposure period for at least oneline of sensor elements subsequent to said first line.
 18. A method asin claim 17, wherein step (y) includes using a look-up table todetermine the timing of said exposure stop signal based upon saidaveraged image signal level.
 19. A method as in claim 17, wherein instep (y) said averaged image signal is used to determine the timing ofan exposure stop signal applied to terminate the exposure period of asubsequent line of sensor elements at least one line removed from theline whose image signal level is averaged in step (x).
 20. A method asin claim 15, additionally including the steps of:(i) coupling said imagesignals from said output point to a memory unit; and (j) decoding saiddataform in a decoder coupled to said memory unit.